Monitoring of the well-being of the fetus inside the uterus is very important and is carried periodically with respect to various parameters of the fetus. One of the important parameters to be monitored is oxygen saturation. Various techniques have been developed to enable noninvasive measurements of oxygen saturation.
For example, U.S. Pat. No. 5,494,032 discloses an oximeter for reliable clinical determination of blood oxygen saturation in a fetus. This technique utilizes a multiple frequency light source which is coupled to an optical fiber. The output of the fiber is used to illuminate blood containing tissue of the fetus. The reflected light is transmitted back to the apparatus where the light intensities are simultaneously detected at multiple frequencies. The resulting spectrum is then analyzed for determination of oxygen saturation. The analysis method uses multivariate calibration techniques that compensate for nonlinear spectral response, model interfering spectral responses and detect outlier data with high sensitivity.
A pulse oximetry based technique for determining the fetal arterial blood oxygenation is disclosed in the following article: A. Zourabian et al., “Trans-abdominal monitoring of fetal arterial blood oxygenation using pulse oxymetry”, Journal of Biomedical Optics, Vol. 5, No. 4, October 2000, pp. 391-405.
U.S. Pat. No. 6,041,248 describes a method and apparatus for frequency encoded ultrasound-modulated optical tomography of dense turbid media. The apparatus includes a function generator producing a frequency sweep signal which is applied to an ultrasonic transducer. The ultrasonic transducer produces ultrasonic wave in a turbid medium. Coherent light from a laser is passed through turbid medium where it is modulated by the ultrasonic wave. A photomultiplier tube detects the light which passes through the turbid medium. The signal from the photomultiplier tube is fed to an oscilloscope and then to a computer where differences in light intensity at different frequencies can determine the location of objects in the turbid medium.
The conventionally used techniques for monitoring the well-being of the fetus inside the uterus utilize measuring the fetal-heart-rate (FHR) by placing sensors on the skin of the mother's abdomen proximal to the fetus. These sensors transmit acoustic waves and provide data indicative of the Doppler shift of an acoustic wave reflected from the fetal heart, enabling calculation of the heart rate based on this shift. A normal fetal-heart-rate (FHR) pattern is usually associated with the delivery of a normal well-oxygenated infant. However, a non-reassuring FHR is not always associated with the delivery of a compromised infant.
In the case of non-reassuring FHR, the fetal blood oxygen saturation level can be measured only post membrane rupture by either fetal scalp sampling, which measures the pH level of the fetal blood, or by attaching a pulse oximeter to the presenting part of the fetal head during labor. Both of these methods are performed following the rupture of membranes where the fetal scalp and/or cheeks can be reached.
Another important procedure to be done to monitor the well-being of the fetus consists of assessing the maturity of fetal lungs, which is one of the major concerns of pre-term deliveries. If the baby is delivered and the lungs are not mature, the baby may develop Respiratory Distress Syndrome (RDS), which can result either in fetal death or in long-lasting periods of repeated respiratory difficulty.
In cases where intervention is considered in the course of pregnancy (such as caesarean section or induction of labor) and there is a need to assess the maturity of the lungs, amniotic fluid is drained. Measuring phospholipids in amniotic fluid as the lecithin/sphingomyelin ratio using the thin-layer chromatography method has been the established clinical procedure for predicting fetal lung maturity. Although it is the clinical “gold standard” method, it remains a time-consuming process, has a large intralaboratory and interlaboratory coefficient of variation, and requires expertise. In addition, the procedure of amniotic fluid drainage itself is invasive and suffers a small risk of abortion. Additional techniques that are used for assessing lung maturity levels include measuring the number of lamellar bodies in a volume of amniotic fluid, measuring the prostaglandin level in amniotic fluid and measuring the fluorescence polarization of a sampled amniotic fluid.
When a fetus is acutely distressed, for example as a result of strangulation by the umbilical cord, the bowel content, meconium, may be passed into the amniotic fluid (AF). Assessment of meconial contamination of AF is important in the management of late pregnancy. It appears in nearly one third of all fetuses by 42 weeks of gestation. In cases where the fetus gasps during delivery, inhaling the sticky meconium into the upper respiratory tract results in partial airways obstruction. Meconium aspiration syndrome occurs in 0.2% to 1% of all deliveries and has a mortality rate as high as 18%. The disease is responsible for 2% of all prenatal deaths.
To date, meconium stained amniotic fluid is diagnosed following the rupture of membranes, when the amniotic fluid is drained. However, in cases where the fetus head is tightly fitted in the pelvis, the amniotic fluid is not drained out resulting in misdiagnosis of the potential harmful outcome to the respiratory tract.